For many applications, diamond is a highly desirable material as a coating on a substrate base material. However, where the coating is subjected to severe mechanical loads, its adherence to the base material may fail and result in spalling away of the coating. This is particularly true for cutting tools which are coated with a thin layer of CVD diamond to improve resistance to wear.
One of the most important base materials for various kinds of both flat and round cutting tools is cemented carbide, such as tungsten carbide. Tungsten carbide is a composite of tungsten carbide particles embedded in a binder matrix of cobalt. This material is particularly useful for cutting tools because of its toughness. The provision of a thin coating of diamond on it would be very advantageous because it would in effect combine the benefits of the much harder diamond with the superior toughness of the tungsten carbide substrate to result in a tool which is both very tough and very hard. However, it has been found that it is very difficult to apply a coating of CVD diamond to tungsten carbide such that it forms both a mechanical and a chemical attachment to the tungsten carbide which will be so strongly adherent that during cutting it will wear off, rather than just spall off the surface. The difficulty in providing an adherent coating of CVD diamond to tungsten carbide is generally attributed to the presence of the cobalt, which at the elevated temperatures needed for the CVD diamond deposition acts as a catalyst for conversion of the diamond to graphitic carbon.
Attempts have been made by others to reduce or eliminate the cobalt from the surface to which the diamond is applied in order to improve the adherence. One approach has been to etch away some of the surface cobalt with acid. However, this may leave a portion of the tungsten carbide grains without sufficient mechanical support to the remaining composite to provide a sound bond for diamond deposited on the surface. Another approach has been to provide an interlayer of material, such as a carbide-forming metal, which will bond to both the base material and the diamond. Titanium may be used for this. However, the presence of an interlayer further complicates the manufacturing process.
There is a need for a process which will permit the deposition by CVD of a diamond coating directly on a substrate, especially tungsten carbide, so that it forms a strong physical and chemical bond with the substrate surface to prevent spalling under severe mechanical loads.